Hyperpolarised gas imaging

What is hyperpolarised gas?

Hyperpolarisation means aligning the nuclei of gases to a much greater extent than that achieved by being in an MRI scanner.

Helium (³He) and Xenon (¹²⁹Xe) can’t be used for MRI scans without polarising them, because the gases are nearly 4 orders of magnitude lower than the density of tissues that would be scanned on an MRI scanner.

The polarising process enhances the MRI signal strength of the gases by 5 orders of magnitude.

The process of hyperpolarisation can align nuclei to nearly 100%. To transform an unaligned nucleus to an aligned nucleus, we can add one quantum unit of angular momentum. Light carries one quantum of angular momentum but as nuclei can’t directly absorb light we use alkali metal vapours.

Rubidium is the best for this, as it has one outer electron and when it absorbs resonant, polarised light, it becomes polarised. This step is known as 'optical pumping'.

When the polarised atoms collide with the ³He or ¹²⁹Xe atoms, they can transfer the polarisation to the gas nuclei through optical pumping and spin exchange.

The gases used, ³He and ¹²⁹Xe, are safe to be inhaled by human subjects. During an MRI scan using hyperpolarised gas, we can see how gas is distributed in the airways.

This MRI technique doesn’t use radiation, but CT does, and it can be repeated without causing harm to a patient. CT can only visualise the first 6 branches of the human pulmonary airway tree. However, a hyperpolarised gas MRI can visualise all 23 branches of the human airways within the lung.

The technique is extremely useful in detecting early stages of a disease as a CT wouldn’t necessarily pick it up, whereas this form of MRI would.

1H MRI

These two images show a proton image of a healthy volunteers lungs (on the left) to a COPD patients lungs (on the right). There isn’t an obvious difference between the two.

³He MRI

These two 3He images show a clear difference between the two set of lungs and the ventilation defects can be clearly seen in the COPD patient’s lungs.

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How a hyperpolarised gas lung image is obtained

We scan several patients on a weekly basis that are taking part in research studies or have been referred by a clinician for clinical purposes using hyperpolarised gas.

The difference between a standard MRI (Magnetic Resonance Imaging) and a hyperpolarised gas MRI is that the study subject or patient will inhale harmless gases, helium and xenon.

Depending on the study the subject is in, the clinical question being asked will depend on the sequences that will be performed.

An MRI sequence is a particular setting of pulse sequences and pulsed field gradients that result in a particular image appearance. Some of these sequences are explained below.

Ventilation

When the gas is inhaled, it travels through the airways and fills the lung air spaces. The MRI reflects the location of the gas at that time point.

This imaging shows the regional distribution of gas within the lung. If the gas doesn’t enter an area of the lung, this means it’s not ventilated and there is a defect.

Diffusion

Hyperpolarised gas present in air spaces within the lung is free to diffuse around the airways and alveoli.

Using this technique determines the structure of the airways of the lungs and allows us to measure alveolar size and the shape of the airway. Diffusion can detect regional microstructural changes.

Perfusion

A perfusion scan involves having contrast dye injected into a vein and this dye highlights areas that receive good blood supply and areas that don’t.

Pulmonary structural

The gas being used for the scan will be polarised in order for it to be used during the scan. Hyperpolarised gas MRI scans can vary in length depending on the sequences being performed. When instructed, the subject inhales the gas from a bag.

They will hold their breathe for a few seconds and they will then be told to breathe the gas away.

If the scan is being used for research purposes, it will be analysed and will be used in the results of the study. If a clinician asked for the scan to be performed for clinical purposes, the images are analysed and put into a special report that our unit creates.

This is sent onto the referring clinician along with a radiologists report so that the clinician has detailed results of the scan in order to assist with further treatment.